A transformer's alternate mode (common name for vehicle mode) has to fit within certain boundaries, and they use holograms and other minor tricks to make this alternate mode look like something similar to the world they're on.
As such, a transformer with a car altmode may be able to choose what kind of car within a range of cars he transforms into, but he can't become a truck, a train, a ship or a plane.
Transformers are Robots in Disguise. They don't transform into an exact replica of their altmode, they transform into something close enough for them to put on a disguise to complete the package.
Q. At the BotCon 2008 Hasbro Commentary viewing of the live-action Transformers film, an early exchange between Hasbro and Michael Bay was brought up: Michael Bay questioned the logic of damaged Transformers turning into pristine vehicles, and the Hasbro response to him was along the lines of "the Transformers don't actually turn into the vehicles, they're just mimicking them". Is this concept something that the "movie" universe is still operating under?
A. Transformers across all of our expressions 'mimic' vehicles. They are "robots in disguise" even when they are in vehicle mode. The inherent ability of Transformers to convert their bodies from space to space combined with their ability to create holograms fulfills the illusion of a pristine car in vehicle mode.
As always, we try to remain true to the characters and put out the best toys as all possible at all times.